1. Field of the Invention
The present invention relates to an oxidizing agent useful for polymerization of high conductive polymer, more particularly a kind of metal ion complex formed of metal ion salts having oxidizing capability and one or a plurality of nitrogen-containing compounds.
2. Description of the Related Art
Since it was found that doped polyacetylene can exhibit electrical conductivity, considerable efforts have been invested in the development and application of new types of electrically conductive conjugate polymer. Currently most commonly applied conductive polymers include polypyrrole, polyaniline, polythiophene and the derivatives of those compounds and have been used in anti-static coating, electrolytic capacitor, printed circuit board, solar collector and light-emitting display.
Conductive polymers are synthesized mainly by electrochemical polymerization or oxidative polymerization. Electrochemical polymerization is achieved by dissolving monomers in a solution containing electrolytes and passing through current to form conjugate polymer at the anode. The electrolytes in the solution would undergo doping reaction in the process to endow the conjugate polymer with electrical conductivity. Oxidative polymerization entails oxidizing the monomers with oxidizing agent to cause polymerization and subsequently adding in dopant in gaseous or liquid phase to give the resulting polymer electrical conductivity. The electrochemical polymerization process is known to produce polymers with excellent conductivity, and can get free standing films of certain conducting polymers such as polypyrrole. However the application of this method is severely hampered by the requirements of processed object being electrically conductive and having high oxidation potential. Oxidative polymerization does not have such requirements. In this process, monomers, oxidant, dopant and solvent are mixed, and coating or impregnation can be carried out before monomers are massively polymerized into conductive polymer.
The practicality of oxidative polymerization and the difficulty of its processing technology are governed by the stability of monomer-oxidant mixture. The mixing of monomers and oxidant will induce the polymerization of monomers which causes the viscosity of the mixture to rise, making it difficult to be coated or impregnated onto the processed object. Thus how to obtain stable monomer-oxidant mixture and high conductive polymer are the focus in the research of oxidative polymerization.
In the example of making solid-state electrolytic capacitor, to reduce the reaction rate of oxidant-monomer mixture and prolong the processing time of the mixture, Friedrich Jonas et al. in their U.S. Pat. No. 4,910,645 discloses the use of large amount of solvent to dilute the concentration of monomers to below 10 wt % so as to slow down the reaction rate of monomer-oxidant solution at room temperature. But by impregnating a capacitor element with monomer-oxidant mixture, only small amount of conductive polymer is obtained in one impregnation, while the remaining part consists of residual reactants and large amount of solvent. This proposed process needs to go through 16 cycles of impregnation and polymerization to produce sufficient conductive polymers to fill the gap between the positive and negative foils of capacitor element (U.S. Pat. No. 6,136,176). Thus using conductive polymer as an electrode of solid state electrolytic capacitor has the drawbacks of complex process and high manufacturing cost.
Friedrich Jonas et al. also disclose in U.S. Pat. No. 4,959,430 a thiophene derivative—3,4-ethylenedioxythiophene which has lower polymerization rate after mixing with oxidant under room temperature and the resulting (poly(3,4-ethylenedioxythiophene) has excellent conductivity and thermal stability. But the polymerization rate of this monomer at room temperature when mixing with oxidant is still controlled by the concentration of oxidant. High oxidant concentration will affect significantly the room-temperature stability of mixture, hence restricting the processing conditions of oxidative polymerization.
Philip M. Lessner et al. in U.S. Pat. No. 6,046,899 discloses a complex formed by the mixture of oxygen-containing organic compound with specific low boiling point, e.g. tetrahydrofuran, and Fe (III) oxidizing agent, which reduces the oxidizing power of oxidizing agent so the mixture of monomer and oxidizing agent can be stably preserved for a longer period of time. After the capacitor element is impregnated with this mixture, the low-boiling solvent can be evaporated under high temperature, and monomers and oxidizing agent can react to form conductive polymer. The oxygen-containing compound disclosed in the aforesaid patent, e.g. tetrahydrofuran, acts as a polymerization retardant. But the complex formed of such compound and Fe (III) oxidizing agent does not have much effect on the oxidative capability of ferric ion, hence having limited effect on stabilizing the mixture of monomer and oxidizing agent at room temperature. This process fails to effectively simplify the process for preparing conductive polymer solid state electrolytic capacitor and reduce its cost.
Reuter et al. in US patent 2005/0013094 reveals a polythiophene conductive polymer polymerized by using a thiophene and a metal salt (such as iron(III), copper(II), chromium(VI), cerium(IV), manganese(IV), manganese(VII) and ruthenium(III)) as an oxidizing agent in the presence of a nitrogen-containing compound including the amide compounds (such as methylacetamide, dimethylacetamide, and dimethylformamide) as a solvent. Because the nitrogen-containing compound is used as a solvent, the molar ratio of the nitrogen-containing compound to a metal ion of the metal salt is so high that the monomer for forming the conductive polymer can not be polymerized. Therefore, the metal salt and the nitrogen-containing compound disclosed in US patent 2005/0013094 is not suitable to be used as an oxidizing agent for retarding the polymerization of the monomer for forming the conductive polymer at room temperature. Besides, US patent 2005/0013094 fails to disclose the molar ratio of the nitrogen-containing compound to a metal ion of the metal salt for retarding the polymerization of the monomer for forming the conductive polymer at room temperature.
Luebben et al. in US patent 2003/0088032 reveals block copolymers prepared by polymerization of intrinsically conducting polymer blocks having heteroaromatic monomers and non-conducting blocks of various chemical structures, wherein heteroaromatic monomers may have a urethane residue. Also, it discloses that chemical polymerization can be performed in the presence of an oxidizing agent such as inorganic salt of iron (III), chromium (IV) and copper (II). However, Luebben et al. fails to disclose the molar ratio of the nitrogen-containing compound to a metal ion of the metal salt for retarding the polymerization of the monomer for forming the conductive polymer at room temperature. Moreover, the urethane residue having amide group in US patent 2003/0088032 is used as a polymerizable monomer, not used as a retarder for weakening the oxidizing strength of metal ion at room temperature.
Lellouche, Jean-Paul in US patent 2006/0047067 discloses a novel polymerizable monomer having a residue capable of reacting with a nucleophile group. Said residue may include —NR1R2, wherein R1 and R2 independently represent H, —N-succinimide, —N-phthalimide, pentafluorophenyl, biotin, aromatics, sugars or 1,2-/1,3-amino alcohols. Also, it discloses a nitrogen-containing compound, such as dimethylformamide. Nevertheless, Lellouche, Jean-Paul does not disclose that the nitrogen-containing compound can be combined with a metal salt within specific molar ratio to be used as an oxidizing agent for retarding the polymerization of the monomer for forming the conductive polymer at room temperature.
Thus the focus of efforts on oxidative polymerization is to develop a process that effectively inhibits the room-temperature polymerization rate of monomer-oxidant mixture without at the same time affecting the conductivity of resulting polymer or adding to process difficulty.